This invention relates to a polycarbonate resin composition for optical use, and in particular, a polycarbonate resin composition which can manufacture an optical disc substrate which has excellent transparency, and is free from internal defects.
Because of its excellent transparency, heat-resistance, and dimensional stability, polycarbonate is often used as a molding material for optical products, including optical discs, lenses, and prisms. These molding materials for optical use generally must be free from internal defects (nicks) in addition to being colorless and having high transparency qualities.
On one hand, polycarbonate is usually manufactured through the reaction of a dihydroxydiaryl such as bisphenol A with phosgene in the presence of an alkali, and in a solvent mixture comprising water and hydrocarbon halide. However, in the case of polycarbonate for optical use, the molecular weight of polymers must be adjusted to a relatively low level in order to improve its moldability, and the mixing of the solvent and by-product salt in the polymer must be suppressed as much as possible by sufficiently refining the product polymer and preventing the attachment of foreign substances.
When an optical disc substrate is manufactured from polycarbonate resin obtained by the aforementioned method, by injection molding, for example, the disc has no problems immediately after molding, but when subjected to high temperature and humidity for an extended period of time, this causes fine point defects of several ten micron diameter to develop in the disc.
Such point defects are usually very small, and they occur infrequently, but when used as an optical disc substrate, they affect the reliability of recorded information, such as the development of signal read errors; therefore, it is important to suppress the occurrence of defects whenever practical. The cause of the occurrence of point defects is not clear, but may be attributable to partial hydrolysis in the polycarbonate resin. Attempts have been made to prevent this problem by mixing various types of stabilizing agents into the polycarbonate resin, but no adequate method has been found to date that can suppress the aforementioned defects without damaging impairing optical functions.
It is an object of this invention to provide a polycarbonate resin composition for optical use mixed with specific types of compounds to form optical products with an infrequent occurrence of the point defects, without coloring, and having good transparency, even when they are stored under high temperatures and humidity for an extended period of time.
It is another object of this invention to provide an optical disc substrate which is excellent in transparency, free from coloring, and causing few point defects.